The incidence of obesity has increased dramatically during recent decades. Consequently, obesity and its comorbidities, most notably insulin resistance and Type 2 diabetes (T2D), constitute a serious threat to global health. Of particular concern is the growing epidemic of childhood obesity and the increases in obesity-related metabolic disorders in children. It is now clear that obesity is accompanied by a low-grade systemic inflammation, and that this chronic inflammatory state exacerbates lipid accumulation and contributes to the development of obesity-associated comorbidities. Despite these advances in our understanding of obesity- associated abnormalities and how chronic inflammation contributes to its comorbidities, much remains to be learned regarding the factors and the molecular events that initiate and promote chronic inflammation in obesity. Preliminary studies performed in the PIs' laboratories, using mouse models of obesity, have provided strong evidence indicating that 1) distinct dietary lipids differentially influence the functional activity of a subset of immune cells called Natural Killer T (NKT) cells, which react to lipids and play a critical role in regulating immune and inflammatory responses; 2) a high fat diet (HFD) skews NKT cells towards a proinflammatory phenotype cytokine production profile; 3) NKT cell deficiency partially protects against HFD-induced weight gain, fatty liver, and insulin resistance; 4) NKT cell deficiency ameliorates inflammatory status and macrophage content in tissues; and 5) chronic stimulation of NKT cells by the NKT cell specific agonist - galactosylceramide exacerbates HFD-induced fatty liver and insulin resistance. These findings suggest a scenario in which elevated lipids in obesity cause NKT cell dysfunction, which in turn plays a pathogenic role in obesity-related metabolic diseases. As such, NKT cells represent attractive targets for interrupting the vicious cycle between metabolic disorders and inflammation during obesity. The proposed project will test the hypothesis that NKT cells, by responding to changes in lipids through interactions with antigen-presenting cells, contribute to the development of chronic inflammation, thereby exacerbating obesity and its metabolic consequences, in particular insulin resistance and T2D. We propose 4 integrated Aims: Aim 1: To investigate the cellular and molecular mechanisms by which lipid excess modulates NKT cell function in obesity; Aim 2: To determine the role of NKT cell dysfunction in initiating and/or exacerbating chronic inflammation, macrophage polarization, and systemic insulin resistance in obesity; Aim 3: To evaluate the impact of distinct NKT cell specific exogenous agonists on obesity-triggered inflammation and insulin resistance; and Aim 4: To assess the functional activity of NKT cells in obese children. Completion of the work described in this proposal will provide novel insight into fundamental NKT cell biology and the development of obesity-triggered inflammation, and will provide a strong rationale for developing safe and effective NKT cell-based preventive measures and therapies for correcting excess adiposity and its metabolic consequences, in particular T2D. Page 1